I currently have a 75g freshwater that I am currently redecorating I had a bunch of plants in the tank at one point but they weren't appropriate for the lighting that I have. So I'm going to be adding plants that can tolerate the low wattage lights that I have. I have eco-complete along with regular gravel in the tank right now that has been there since June of 07. Combined the gravel is about four inches deep.

My question is should I add more eco-complete? Or should I maybe think about adding some laterite to the gravel that I have? Or leave it alone?

The other thought I have if you all say to add more eco-complete is, won't it turn the tank completely black or at least grayish and will this hurt the fish?

I can see no reason why you would need to add more eco complete. It will indeed make a mess of your tank, but it shouldn't cause any harm for the fish.
Please remember that too much fertilizer can cause as much harm to plants as not enough. A 4 inch substrate is going to be very difficult to keep clean, meaning watch your nitrate levels. Long term you may find you have a lot of nitrate buildup... and that will harm fish.